The invention relates to a method of manufacturing a charge-coupled image sensor arrangement of the buried channel type in which a region of a semiconductor body of a first conductivity type adjoining a surface is provided with a number of adjacent surface zones of the first conductivity type which form the buried channels of the charge-coupled device, and with channel-bounding surface zones of a second, opposite conductivity type situated between the surface zones of the first conductivity type and extending from the surface to a greater depth than the surface zones of the first conductivity type into the semiconductor body to below these latter surface zones, separating them at least partly from adjoining portions of the semiconductor body of the first conductivity type. The invention also relates to a charge-coupled image sensor arrangement obtained by this method.
Such a method and a device manufactured by this method are known from inter alia from the European Patent Application 143.196laid open to public inspection.
In these devices, which are usually of the n-channel type, the n-type semiconductor body with which the manufacture starts forms a drain region for excess charge carriers which may be generated in the image sensor arrangement as a result of overexposure. In comparison with devices in which the drainage of excess charges takes place via drainage channels situated at the surface between the channels of the charge-coupled device, this vertical drainage has the advantage that the light sensitivity of the image sensor arrangement is not reduced. The overflow potential barrier between the n-type CCD channels and the n-type substrate are formed by the p-type channel-bounding zones provided on either side of each CCD channel, which zones diffuse from the sides of the CCD channels to below the CCD channels. This diffusion is usually carried through so far until the n-type CCD channels are completely separated from the n-type substrate by a p-type region. Doping and thickness of this p-type region are such that the region can be depleted over its entire thickness without breakdown. In an alternative embodiment, a narrow n-type region may be left below the CCD channels which acts as a vertical drainage channel.
The starting material for the manufacture of the known device is an n-type substrate at whose surface the p-type channel-bounding zones are provided by means of a mask, from which zones a continuous p-type region is formed by means of diffusion with constrictions where two zones diffusing towards one another adjoin one another. Subsequently, a second mask is used to provide the n-type surface zones for the CCD channels in such a way that the centers of these zones lie above or at least substantially above the said constrictions.
Two masks are required for the two subsequent doping steps in this known method, the second mask having to be aligned as accurately as possible relative to the pattern of p-type zones provided during the first doping step. The reproducibility of the device is reduced as a result of the spread in accuracy with which the n-type and p-type zones are provided relative to one another. In addition, the known method described here has the result that the maximum doping concentration is at or close to the surface in the channel-bounding p-type zones. It is often necessary, furthermore, to provide an additional shallow p-layer in the channel-bounding region by means of a third mask for the drainage of generated holes. It will often be desirable in view of the photosensitivity for the maximum doping concentration to be at some distance away from the surface, the doping concentration decreasing from this point towards the surface.